The present invention relates to the coherent interaction of optical radiation beams with ions or molecules in solids, and to the choice of propagation direction and light polarization relative to the crystal symmetry axes of the solid, and more particularly to optimize the optical-electronic interaction effects in materials with generalized crystal symmetry.
A variety of optical-electronic applications are based on the coherent interaction of optical radiation beams or fields with ion-doped or molecular crystals of various types; these interactions include optical coherent transients, spectral hole burning, and spatial-spectral holography (also called time- and space-domain holography). Devices based on these concepts are used in optical data storage, real-time optical signal processing, quantum computers, and other coherent computers where the coherent interaction of multiple radiation beams is enhanced, enhanced data erasure in coherent computers, and optical data routing and have applications to computers, communications networks, the Internet and other networks, time delays in RADAR, and numerous other applications.
Natural and synthetic optical materials have a wide range of potential crystal lattice symmetries. (A well-known catalog of all crystal space groups is the International Tables for Crystallography, Edited by Theo Hahn, published by the International Union of Crystallography, D. Reidel Publishing Co.) Within these materials, active ions or molecules occupy crystal xe2x80x98lattice sitesxe2x80x99 that can be cataloged into subsets, with members of each subset having identical surroundings and having similar resonant frequencies for coupling to optical radiation (the members of each subset are said to be xe2x80x98crystallographically-equivalentxe2x80x99); each crystallographically-equivalent subset of lattice sites may contain ions or molecules with a finite number of different spatial orientations. The optical transitions of electrons in the ions or molecules can be described by two quantum energy levels and a transition dipole moment. In general, these transition dipole moments have a plurality of different spatial orientations, according to the different orientations of the crystallographically-equivalent sites noted above. Light beams, on the other hand, must have single optical propagation directions and polarization states relative to the crystal, with the consequence that the light polarization will have a plurality of different spatial relationships with otherwise identical ions or molecules.
When resonant coherent interactions occur, the interaction of the optical field and the two-level quantum systems can be characterized by the optical Rabi frequency:       ω    R    =                    p        _            ·                        E          O                _              ℏ  
where {overscore (p)} is the electric dipole moment with components pi= less than 1|pi|2 greater than  and {overscore (E)}0 is the optical electric field vector. (Similar expressions apply for magnetic dipoles and magnetic optical fields. In extreme situations a power-broadened version of the above equation applies.)
The Rabi frequency is determined not, only by the magnitudes of the transition dipole moment and of the optical field, but also by the projection of one onto the other (vector projection or scalar product). Consequently, when arbitrarily polarized radiation is propagated through such materials, the coherent interaction of the field and the crystalline matter will induce macroscopic polarization oscillations at a plurality of different optical Rabi frequencies.
The presence of multiple optical Rabi frequencies generally reduces the effectiveness of the optical-electronic device due to consequent complex transient material polarization behavior and the optical interference or beating of the associated optical signal amplitudes radiated by the material. Such interference, for example, can in turn limit the optical-electronic system bandwidth and hence the response time and data handling capability in the optical-electronic application. The interference may also reduce the optical diffraction efficiency, i.e., the signal selection or deflection efficiency in such devices as optical data routers for optical communications networks and wavelength-division multiplexing systems.
To avoid the deleterious effects of this multiple frequency interference, while still being able to optimize other system parameters, it is necessary to design a procedure for obtaining xe2x80x98single-Rabi-frequencyxe2x80x99 behavior in a generalized situation. The small group of materials that have only a single site orientation can readily exhibit single frequency behavior. Here, However, we show that a wider range of materials, including multi-site materials, can exhibit single-Rabi-frequency behavior under the conditions that we have discovered.
The crystalline compound, Y3Al5O12 (yttrium aluminum garnetxe2x80x94xe2x80x9cYAGxe2x80x9d), which has been used by several research groups for device demonstrations, is a particularly complicated optical material, and its behavior serves to illustrate the problems arising from the interference effects described above, and also to illustrate our invention. In past applications, light was propagated along the so-called crystallographic  less than 111 greater than  direction of YAG, a propagation direction that does not yield single-Rabi-frequency behavior. There are one hundred sixty (160) ions per unit cell of the YAG lattice (the unit cell is the fundamental building block of the crystal). When rare earth ions are substituted as active ions for yttrium at the dodecahedral lattice sites, there are six (6) crystallographically-equivalent sites, each with a differently-oriented local environment. Hence, there are six different directions for the individual transition dipoles of the active rare earth ions. For an arbitrary optical propagation direction and for arbitrary optical polarization, there will be six different Rabi frequencies.
In principle, one could eliminate the degradation in performance arising from the presence of multiple Rabi frequencies by choosing a different material with an appropriately high symmetry that restricts the sites to a single orientation. In general, though, that high-crystal-symmetry approach to obtaining a single Rabi frequency does not work for device applications, since one must simultaneously optimize many other material properties, including the optical coherence or dephasing time, inhomogeneous optical line broadening, transition probability, persistence of spectral hole burning, dependence of all of these properties on the applied magnetic field, and dependence of all of these properties on temperature. Satisfying all of these demands in a single material is at best a difficult challenge, even when no other restrictions on material selection exist (such as the restriction to a single site orientation). This xe2x80x98high-symmetryxe2x80x99 approach has so far proven to be impractical. Conventionally, the optical material had to be chosen from a small subset of available materials, most or all of which do not have a single set of identically-aligned and oriented crystallographically-equivalent dipoles; that represents a sacrifice in potential bandwidth, diffraction efficiency, and performance.
The following example shows the difficulty of the high-symmetry single-site-orientation approach. The choice of a single-site material would make it relatively simple to achieve a single optical Rabi frequency in the transmission of a radiation beam or field, and could thereby increase the effectiveness of the device, but it also significantly reduces the optical transition probability in many cases and even reduces it identically to zero in many cases of interest. It also restricts the choice of materials that may be used to a very small fraction of the totality of optical materials that might be otherwise considered for use in the particular application. Since many interesting optical materials with multiple site orientations have other characteristics that are superior to those of materials with the high crystal symmetry necessary to give a single site orientation, it is advantageous to solve the single Rabi frequency problem in a generalized way, so that optical-electronic devices can benefit from the use of materials with multiple site orientations specifically and from a far wider range of materials generally.
Accordingly, it is an object of the present invention to provide a technique for eliminating the deleterious effects of multiple Rabi frequencies on the speed and bandwidth characteristics of optical-electronic interaction effects for materials with generalized crystal symmetry.
It is another object of the present invention to provide a technique for reducing material polarization interference in optical-electronic interactions.
It is a further object of the present invention to provide a technique for optimizing optical transition probability in optical-electronic interactions for a wide range of optical materials.
It is a further object of the present invention to provide a technique for optimizing diffraction efficiency in optical-electronic interactions for a wide range of optical materials.
Additional objects, advantages, and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description, as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of significant utility.
In accordance with the present invention, to reduce undesirable interferences and thereby increase the effectiveness of optical-electronic interactions, a beam of radiation, e.g. a coherent light beam, (or multiple beams of radiation) is propagated through a material having a generalized crystal symmetry with specific light propagation direction and polarization state specified relative to the conventional axes of crystal symmetry of the material (linear or elliptical polarization). Since the material has a generalized crystal symmetry, the material""s transition dipoles will have several independent transition directions; that is, the material will have a crystal lattice structure with a plurality of unaligned, differently-orientated dipoles at crystallographically-equivalent sites.
The invention involves a procedure for determining a suitable optical propagation direction and polarization state that projects equally on the respective directions of the transition dipoles, or more typically a subclass of the dipoles, and which is orthogonal to the respective directions of any remaining transition dipoles, i.e. all dipoles of the original subset not within the subclass of dipoles. The radiation beam is polarized, linearly or otherwise, relative to the axes of symmetry to equally project onto each of the dipoles within the subclass of dipoles. Beneficially, the propagating radiation beam is polarized identically with respect to each of the dipoles within the subclass and orthogonally to dipoles outside the subclass of transition dipoles, and accordingly, equally projects onto each of the dipoles within the subclass of transition dipoles.
The propagation of the polarized radiation beam through the material actively excites ions in the subclass of dipoles so as to induce them to oscillate cooperatively. The dipoles of the ions outside the subclass preferably are not oscillated. Advantageously, each of the dipoles in the subclass are oscillated at the substantially same Rabi frequency and have a substantially equal transition intensity.
The relationship of the propagation direction and polarization to the axes of crystal symmetry will change dependent upon the selection of dipoles forming the subclass of dipoles. In any event, the subclass of the dipoles is selected such that there exists a special direction, specified relative to the conventional axes of crystal symmetry, that projects equally on the respective transition dipoles within the subclass and is orthogonal to the respective transition dipoles of the remainder of the dipoles.
According to the invention, a system for propagating a beam of radiation through a material having a plurality of unaligned, differently-orientated crystallographically-equivalent transition dipoles includes a monochromatic frequency-agile radiation emitter (or several emitters) for emitting a beam (or multiple beams) of radiation along a path towards the optical-electronic material. An optical controller or encoder for each beam made up, for example, of an acousto-optical, electro-optical, or other modulator or combination of modulators imposes amplitude or phase information on the beam of radiation (or prepares the beam to manipulate another of the several radiation beams). A possibly-two-dimensional deflector makes adjustments in the beam direction as may be required by the optical-electronic device application. Input optics direct the radiation beam(s) to the crystalline material where the radiation-material interaction critical to device performance occurs. A polarizer is configured (or polarizers are configured) to polarize each emitted radiation beam in a direction such that the radiation beam polarization(s) has (have individually) substantially the same projection with respect to multiple dipoles making up a subclass of transition dipoles. In some cases, the typically very small angles between multiple incident radiation beams can be freely varied while maintaining the desired single Rabi frequency behavior (the garnet example illustrates that); in other cases the small angles typically involved between several beams mean that the conditions can be met simultaneously to a good approximation. Beyond the optical-electronic material, there are output optics and an array of radiation detectors or other independent receiving channels such as optical fibers for the signals.